Interferometers, such as those of the Michelson Fourier-transform infrared (FTIR) variety, often incorporate a moving mirror that is used to vary the phase of various wavelengths of a beam of analytical radiation, e.g., infrared, that passes through a sample of material. The operation of such a Michelson interferometer FTIR spectrometer system is critically dependent upon the accurate determination of the interferometer's moving mirror position. Motion of the interferometer moving mirror is normally tracked by a positioning monochromatic light beam (usually from a laser) operating in parallel with the analytical radiation beam in the spectrometer, with the monochromatic light beam also entering the interferometer. Position information on the moving mirror is obtained by counting the fringes produced by the monochromatic light beam as it travels through the interferometer. The monochromatic light beam is made to strike an opto-electronic detector as it exits the interferometer, producing a sinusoidal output signal as the mirror moves. The sine wave has a zero crossing each time the mirror moves one-fourth of the wave length of the light of the monochromatic beam. These zero crossings may then be counted and used as the "yardstick" by which mirror position is determined.
Though this method of counting fringes in a monochromatic light beam determines mirror position to a very high degree of precision, this method requires large electronic counters to hold the many thousands of counts that are generated. When combined with the necessary timing information to determine mirror velocity and control the linear motor that drives the mirror, the implementation of these functions requires extensive hardware.